RMV math…

[Gareth J]

If my SAC on a 12 litre is 2 bar/min, then I can look at my depth, say 30m (~4 ATA), and know that my pressure should drop at 8 bar/min. If It is changing 10 bar/min I am working harder than normal, etc.

But for a SAC of 2 bar/minute to be useful, you need to know the cylinder size, and you have to recalculate for each cylinder size.
However, it does make far more sense than saying SAC is based on your depth.
Knowing you expect to use 8 bar a minute during your dive at the expected depth is a useful figure.

 

[L13]

I can understand RMV being misleading, i.e it could relate to the surface, or to a specified depth.
But SAC is not ambiguous, it is a rate of gas consumption at the surface in the units specified.

Actually, with your definition, you aren't measuring gas consumption at the surface. You measure gas consumption at depth and then calculate what the gas consumption would be if you breathed equally hard at the surface. But the actual volume of the gas you consumed if taken to the surface would be much higher.

SAC, with your definition, is not actually consumption. It is RMV. SAC with the other definition from yours, is actually consumption, but is using Surface to explain the units, not the location it is "measured at".

 

[scubadada]

But for a SAC of 2 bar/minute to be useful, you need to know the cylinder size, and you have to recalculate for each cylinder size.
However, it does make far more sense than saying SAC is based on your depth.
Knowing you expect to use 8 bar a minute during your dive at the expected depth is a useful figure.

That's right, SAC is cylinder dependent, RMV is not. That's probably enough. You are from the UK, I am from the US, we speak different languages. I also wish the US would have gone metric, but it didn't.

This topic has been discussed many, many times. I don't think we have ever come to a universally accepted definition.

 

[Gareth J]

Actually, with your definition, you aren't measuring gas consumption at the surface. You measure gas consumption at depth and then calculate what the gas consumption would be if you breathed equally hard at the surface. But the actual volume of the gas you consumed if taken to the surface would be much higher.

SAC, with your definition, is not actually consumption. It is RMV. SAC with the other definition from yours, is actually consumption, but is using Surface to explain the units, not the location it is "measured at".

My 15l/min SAC is the surface gas consumption rate.

It can't be higher at the surface than at depth.

For every 10m of depth (33ft), pressure increases 1 bar (1 Atmosphere, 14.7psi).

So a SAC of 15l/min = 30l/min @ 10 m (33ft), 45l/min @ 20m (66ft), 60l/min @ 30m (99ft) 75l/min @ 40m (132ft), 90l/min @ 50m (165ft)

This is independent of cylinder size.

Bar per minute would relate to cylinder size!

**** Full disclosure, I don't often dive OC. 15l/min is an easy number to use, my normal SAC is lower, but this gives a margin of error and makes calculation easy.
Bailout numbers are higher for the first part of the bailout calculation, 50l/min. Stress increases gas consumption significantly, Bailing out from CCR to OC is highly stressful. Once at the first point of safety (normally the first stop), the stress level drops significantly.

 

[scubadada]

My 15l/min SAC is the surface gas consumption rate.

It can't be higher at the surface than at depth.

For every 10m of depth (33ft), pressure increases 1 bar (1 Atmosphere, 14.7psi).

So a SAC of 15l/min = 30l/min @ 10 m (33ft), 45l/min @ 20m (66ft), 60l/min @ 30m (99ft) 75l/min @ 40m (132ft), 90l/min @ 50m (165ft)

This is independent of cylinder size.

Bar per minute would relate to cylinder size!

Yes, that is what I call RMV. You must know your avg RMV (SAC), did you vote in the poll? Average Gas Consumption The topic of SAC/RMV was discussed extensively there also, likely, with the same outcome

Maybe this should be a poll?

 

[Gareth J]

Yes, that is what I call RMV. You must know your avg RMV (SAC), did you vote in the poll? Average Gas Consumption The topic of SAC/RMV was discussed extensively there also.

I probably didn't vote in the poll.

Finally a consensus

We are two nations separated by a common language (and a F***ing great Ocean) - to quote a few people!!


In diving, its not helped by different units of measurement ---- as NASA has found out a few times!!!

 

[scubadada]

Hi @Gareth J

What do you call pressure/time/atm, bar/min/atm or psi/min/atm if it is not SAC? Do you use the term RMV for anything?

 

[inquis]

if I have a higher SAC than they do, they can calculate the appropriate turn pressure!!

Never use SAC estimates to plan a turn pressure. (Consider what happens when your actual SAC is worse to see why.)

The proper method is to determine the smallest available volume out of the lot of you, and everyone converts that same volume to pressure in their own particular tank. The person who has the actual worst SAC during the dive will simply hit turn pressure first.

 

[Gareth J]

Hi @Gareth J

What do you call pressure/time/atm, bar/min/atm or psi/min/atm if it is not SAC? Do you use the term RMV for anything?

I (we) use bar (pressure), minutes (time)
SAC - Surface Air Consumption.

RMV seems to be more a (North) American thing.

 

[Gareth J]

Never use SAC estimates to plan a turn pressure. (Consider what happens when your actual SAC is worse to see why.)

The proper method is to determine the smallest available volume out of the lot of you, and everyone converts that same volume to pressure in their own particular tank. The person who has the actual worst SAC during the dive will simply hit turn pressure first.

SAC is used to calculate how much Gas you expect to use in a given time. You calculate turn pressures based available gas.

If your breathing rate is higher than expected, then you will reach the turn pressure earlier than expected. So you will turn for home early.
This does not account for a higher workload on the return leg than the out leg. However, it depends on the dive type and nature. A more technical dive, then you allow 1 third out, 1 third back. If the workload is high coming back, then you have an additional third to use (unless you are sharing air - then your day has got a whole lot worse).

This is reasonably basic stuff

However. If you are doing a penetration dive, or a technical dive, your turn pressure may need to be earlier than the first third of gas, if you are going to accommodate a return at the furthest part of the dive sharing air with a buddy that has a higher gas usage rate than you. (That doesn't even take account of the higher breathing rates related to stress!!).
If my buddy knows my SAC then he/she can add that to his/her gas calculations to determine a turn pressure that ensures sufficient gas to get as safely out of the water. (Or to determine that a larger set of cylinders, or additional cylinders would be more prudent.)